I noticed that some of the more frequently used MO tags are formatted according to a certain pattern, e.g.:
nt.number-theory
pr.probability
lo.logic
fa.functional-analysis
Why are the tags formatted ...

I posted a question and tagged it "infinite-sequences". A high-reputation editor added a more general tag: ca.analysis-and-odes. Now, in principle, having a more general tag is fine---but this tag is ...

The tag system on MathOverflow is designed in such way that each question should contain top-level tag; these are the tags which correspond to subject areas on arXiv and generally have format such as ...

TL;DR: Is there a tag which is intended (only) for ordinary differential equations? If not, should there be one?
The situation with the tags related to ODEs seems a bit unclear to me. AFAICT there is ...